ABSTRACT
The mixed-ligand dithiolene complex [Pt(Bz(2)pipdt)(dcbdt)] (1) bearing the two ligands Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C(2)S(2)PtS(2)C(2) dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C(2)S(2) units exhibit a significant difference in Bz(2)pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz(2)pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (µß(0) = -1296 × 10(-48) esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes.
ABSTRACT
In this paper, we present a theoretical study based on DFT methods using functionals implemented in Gaussian03, to obtain geometry optimizations, harmonic frequencies, IR intensities and Raman scattering activities of the triad [Ni(tdas)2]z [z = 0 (); z = 1- (); z = 2- ()] and of [Au(tdas)2]- (3) with the aim of elucidating the nature of the bonding in these complexes and checking whether C-C stretching vibrations are suitable spectroscopic markers to assign the 'innocent-non-innocent' character of the ligand. Geometry optimization at the density functional theory level was performed, and geometrical parameters were obtained in good agreement with the experimental structural data for the Ni complexes. Following the geometrical changes upon reduction, the Ni-S, N-S and C-S distances undergo a small increase, while the C-C length does not change significantly. Thus, unlike what is generally found for non-benzenoid dithiolenes, no shift of the nu(C-C) vibration is expected on reduction as observed for 1 and 2. The vibrational spectra are very accurately reproduced by the calculations. The less satisfactory agreement obtained for the gold complex is probably due to the deviation of the structurally characterized sample from the D2h symmetry obtained in the calculations, and/or overestimation of the Au-S distances, typical of the DFT functionals used. Calculations indicated that the electroactive orbital, HOMO in and , SOMO in and LUMO in , is a highly delocalized pi-orbital with a predominant S-ligand character, and that the total spin density in is delocalized over the ligands and the nickel atom, in agreement with EPR data. From the obtained results we can assign a non-innocent character to the tdas ligand and show that the C-C stretching vibrations are not suitable spectroscopic markers to reflect this feature.
